A water-absorbent resin is widely used in recent years in a field of an absorbent article including hygienic materials such as disposable diapers and sanitary napkins; agricultural and horticultural materials such as a water-retention agent and a soil conditioner; industrial materials such as a water blocking agent and a dew condensation prevention agent. Although many kinds of water-absorbent resins have been known according to their uses, a water-absorbent resin made of a polymer of a water-soluble ethylenically unsaturated monomer is mainly used.
In a field of an absorbent article, particularly a hygiene product, in recent years, an absorbent material tends to be made thin so as to enhance comfort at use and portability. Methods for thinning an absorbent material include a method of increasing a ratio of a water-absorbent resin in an absorbent material; and a method of enhancing water-absorption performance (a water-retention capacity and a water-absorption capacity under a load) of a water-absorbent resin.
However, when the ratio of a water-absorbent resin in an absorbent material is increased, there is a concern for an effect on skin (skin rash) caused by unreacted monomers (residual monomers) contained in the water-absorbent resin.
On the other hand, in the water-absorbent resin made of a polymer of a water-soluble ethylenically unsaturated monomer, one option for achieving its high water-absorption performance is to decrease its crosslinking density. In view of easy control of polymerization reaction, in general, a persulfate is often used as a compound to be added in polymerization of a water-soluble ethylenically unsaturated monomer. However, a persulfate simultaneously promotes a self-crosslinking in polymerization reaction, and therefore, a crosslinking density inside a resin tends to increase, and it tends to be difficult to obtain a water-absorbent resin having high water-absorption performance. In order to solve this problem, a method has been proposed wherein a water-soluble azo-based radical initiator is used instead of a persulfate which tends to promote a self-crosslinking (see Patent Document 1).
However, when an azo-based compound is used, a large amount of residual monomers tend to exist in the resulting water-absorbent resin since it is difficult to increase the polymerization rate of a water-soluble ethylenically unsaturated monomer.
Some methods have been proposed for decreasing a content of residual monomers in a water-absorbent resin. For example, the following methods have been known; a method wherein two times or more of divided additions of a radical polymerization initiator to a monomer liquid are carried out to increase an amount of the initiator (see Patent Document 2); a method wherein a persulfate is added to a water-absorbent resin during or after polymerization (see Patent Document 3); and a method wherein a reducing substance is added to a water-absorbent resin before or during drying (see Patent Document 4); and the like.